(1) Field of the Invention
The invention relates to the use of a phase shifting mask to improve resolution in the fabrication of sub-micron integrated circuits and more particularly to the use of a self aligning mask with phase shifting regions, half-tone or partially transmitting regions and non phase shifting transparent regions.
(2) Description of the Related Art
As optical lithography advances to 0.5 to 0.35 microns and below new technologies are needed to improve the resolution of the imaging lens. Phase-shifting photomasks have been used to improve resolution as well as depth of focus. Phase shifting processes are described in U.S. Pat. Nos. 5,045,417 to Okamoto, 5,194,344 to Cathey Jr. et al, 5,194,345 to Rolfson, 5,194,346 to Rolfson et al, 5,208,125 to Lowrey et al, 5,217,830 to Lowrey, and 5,225,035 to Rolfson. The methods used in these phase shifters usually use an opaque material in conjunction with a phase shifting material. U.S. Pat. No. 5,190,836 to Nakagawa et al, describes a phase shifter using reflection techniques.
"LITHOGRAPHY'S LEADING EDGE, PART 1: PHASE-SHIFT TECHNOLOGY," published in Semiconductor International, February 1992, pages 42-47 describes phase shift techniques and includes a rim type phase shifter in the Table on page 45. This rim type phase shifter uses an opaque material in conjunction with a phase shifting material.
This invention uses phase shifters using a partially transmitting material with a phase shifting material to provide improved image resolution and depth of focus. This improved image resolution and depth of focus is important as linear dimensions used in integrated circuit technology call for improved resolution and depth of focus.